With recently increasing concerns about environmental issues, weight reduction has been performed more and more for better fuel efficiency typically in the automobile industry. To meet the requirement in weight reduction, investigations have been made actively so as to allow aluminum clad materials (brazing sheets) for automobile heat exchangers to have a lesser wall thickness and a higher strength. The brazing sheets generally have a three-layer structure including layers of a sacrificial material (e.g., Al—Zn material), a core (e.g., Al—Si—Mn—Cu material), and a filler material (e.g., Al—Si material) laminated in this order. For higher strength, investigations have been made to add magnesium (Mg) to the core, namely to strengthen the core by Mg2Si precipitation
Independently, flux brazing is widely employed for the joining of a brazing sheet to assembly a heat exchanger. The flux contributes to better brazeability, and one containing KAlF4 as a principal component is generally employed.
However, the customary flux, when used in a brazing sheet having a core including a magnesium-containing aluminum alloy disadvantageously adversely affects the brazeability. Upon heating for brazing, magnesium in the core migrates into the flux in the filler material surface and reacts with the flux component to form high-melting-point compounds such as KMgF3 and MgF2. This consumes the flux component and probably causes the disadvantage. To prevent this, a flux composition for a magnesium-containing aluminum alloy should be developed so as to advance the weight reduction typically of automobile heat exchangers.
Under these circumstances, to help a brazing sheet having a magnesium-containing aluminum alloy as a core, there have been made investigations on (1) a flux composition further containing CsF in addition to the customary flux component (see Japanese Unexamined Patent Application Publication (JP-A) No. S61-162295); and (2) a flux composition further containing CaF2, NaF, or LiF in addition to the customary flux component (see JP-A No. S61-99569).
However, the flux composition (1) further containing CsF is not suitable for mass production and is poorly practical, because Cs is very expensive. In turn, the flux composition (2) further containing CaF2 or another specific compound exhibits better fluidity as the flux, because the added compound helps the flux to have a lower melting point. Even this flux composition, however, fails to provide sufficiently better brazeability, because even this flux reacts with magnesium as in the customary technique. Independently, a brazing sheet exhibits better brazeability by applying a flux in a larger mass of coating. However, such larger mass of coating causes higher cost. To prevent this, demands have been made to develop a flux that enables satisfactory brazing with low cost.